1. Field of the Invention
This invention relates to a coding system for a video signal processing apparatus, and more particularly to a coding system for a wireless pay-TV system which provides special, original programs only to subscribers who make regular payments for the service. In the pay-TV system, a television signal is transmitted from a broadcasting station in coded form provided by inversion of the video signal by an encoder and the coded television signal is decoded by decoders at receivers so as to reproduce the television signal on the receivers.
2. Description of the Prior Art
A so-called wireless pay-TV system, to which the present invention is applicable, has been developed to eliminate annoying commercial messages indispensable to ordinary commercial broadcasting and possible degradation of program quality due to various restrictions inherent to the commercial broadcasting. This wireless system is expected to be spread wide because expenses and time required for laying cables can be curtailed and there is no limitation in number of subscribers.
In general, in the wireless pay-TV system, a television signal and an aural signal are encoded so that the system may be applied only to the subscribers of the system, excluding non-subscribers. A conventional coding system of a video code processing apparatus for encoding a television signal is disclosed, for example, in Japanese Patent Application Laid-Open Specification No. 11515/1978. Encoding of a video signal is carried out by inverting the polarity of the video signal according to codes determined by outputs from flip-flop circuits connected to an oscillator. Thus, a code generating section per se substantially determines an encoding mode, and an encode control signal, which finally determines whether the polarity of the video signal should be inverted or should not be inverted, is changed randomly. The minimum cycle of change of the encode control signal corresponds to a time (about 16.67 ms) required for one field and the frequency of the change is approximately 30 Hz.
In this connection, it is to be noted that when the video signal transmitted after inversion by an encoder at a broadcasting station is decoded by a subscriber's decoder, there may be produced a lag in intensity levels between the original video signal and the decoded video signal by various causes due to a deficiency of the entire transmission system, a difference in characteristics between the encoder and the decoder, etc. This lag in the intensity levels appears in every field, causing flicker on a picture. When the flicker has a frequency of about 30 Hz, i.e., when the encode control signal at the transmitting side changes at a frequency of about 30 Hz, it cannot be negligible for visual sensation of a human being and becomes undesirable for a viewer. Furthermore, the flicker leads in deterioration in picture.